How do color indicators show the presence of the substance being investigated?
It depends on the indicator and on the substance.
In acid-base investigations, the indicator changes colour depending on [H+] or [
In other investigations, such as redox, the oxidised and reduced species have different colours, or a complex exists between the indicator and one species, but not another. An example of the redox species having different colours is a potassium permanganate reaction. MnO4- is purple in colour, but the reduced form, Mn2+, is colourless. An example of a complex is ferroin, which is red when combined with Fe2+, but cannot combine with Fe3+, so the colour disappears.
Complexation indicators depend on the presence or absence of complexes. Eriochrome black forms a red complex in the presence of metal ions. When the metal ions are removed, the indicator turns blue. Murexide turns red in the presence of calcium, but is violet when no calcium is present.
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An indicator is usually a weak acid or base where the conjugate is a different color.
So let’s assume you have some acid A-H where A is some large organic structure that has a hydrogen it can release. When the H is still on the structure it is one color (let’s say blue) and when the H is taken off it turns to another color (let’s say maize).
So you end up with this H-A (blue) –> H+ + A- (maize)
When you have this in a strong acid, you will get the blue form, and when it goes into a basic solution it will turn maize.